Ignore docs/index.js when searching with ag/ripgrep/sift (#1848)

* Ignore docs/index.js when searching with ag/ripgrep/sift

This makes it easier to search through the code without getting
irrelevant results.

* Ignore docs/*.js when searching with ag/ripgrep/sift

This makes it easier to search through the code without getting
irrelevant results. I couldn't get a glob (`docs/*.js`) to work with `ag
babylon -l | ag docs`, so I just listed them out instead.

.ignore

@@ -1,2 +1,6 @@
-docs/prettier.min.js
+docs/index.js
+docs/parser-babylon.js
+docs/parser-flow.js
+docs/parser-postcss.js
+docs/parser-typescript.js
 dist